The security problem in multihop wireless networks is more severe than that in wired networks since its transmission media is the unprotected air. In this paper, we show how to increase the effective throughput via carefully choosing the multi-path routing for given source and destination nodes, where we call the total packets from the mesh routers to the gateway nodes that are not attacked by an attacker as effective throughput. We assume that the attacker has limited resources for attacking while attacking a node or a link will incur some certain costs. We show that it is NP-hard to find an optimum multipath routing policy even if the attacking strategy is given a prior. We model the problem as a two-player game between the routing policy designer and the attacker and propose a randomized multi-path routing protocol that achieves good effective throughputs under several possible attacking scenarios. More specifically, we theoretically prove that our routing protocols can achieve an effective network throughput (with packets which are not attacked) within a constant factor of the optimum in the worst case. Our theoretic results are confirmed by extensive simulations studies.